I used to use 5.2 for all grain beers. Stopped using it out of laziness once, and everything has still been fine since. Probably water dependant.
Btw, I would think limestone spring water would be some of the best on the planet at avoiding ph crashes. Its like you already have the oyster shells in your water. If anything your mash ph might tend to be too high, depending on initial water source ph. Might not hold up to a crashing sugar wash, but in my initial attempts at all grain I would assume the clean lime stone water and the buffering from malts would work perfectly. Test and monitor and be ready to react, sure, but I'd avoid over complicating it initially. Good whiskey has been made with the limestone water of appalachia by generations of people who had no idea what ph meant. Not that they were all noble savages, just pointing out that limestone water in and of itself does not mean you will have to monkey about with ph and buffers at all.
But since this is about scotch style whisky, here is a quote from Whisky on the Rocks: Origins of the 'Water of Life'
The primary source of water is rain, but what happens to rainwater before its arrival at the distillery affects its chemistry and thus the uniqueness of the resulting malt whisky. The rain may end up as a stream or river, in a loch or a reservoir, coming from the rock as deep or shallow boreholes, or as a spring high on a hillside.
If it falls on bare mountains made of crystalline rocks it will flow rapidly downhill as streams. This water has little chance to interact with the underlying rocks and often has a low mineral content. It will be acid and soft.
On the other hand if the strata are more permeable, or have many joints and fractures, the rain will percolate into and through the rock, dissolving it and increasing the water's mineral content. Limestones and sandstones, for example, yield water rich in carbonates or sulphates; such waters will be neutral or slightly alkaline and hard.
'Soft water, through peat, over granite' was the traditional and still oft-quoted view of the best water for distilling. Remarkably, out of the 100 or so single malt whiskies, less than 20 use water that fits this description.
Now I assume all modern commercial distillers monitor and adjust their ph to some extent, but I've never heard of one going to the expense of using commercial buffers rather than simple acids and minerals.